1. Field of the Invention
This invention relates to a method for plating a powder by substitution. More specifically, it provides a simple and inexpensive method for plating various powders with a metal having a negative standard electrode potential and especially to plating nickel and/or cobalt.
2. Description of the Prior Art
With a marked advance in the metallurgical industry, the catalytic chemistry, the electronics industry, etc. in recent years, there has been a strikingly increasing demand for various plated powders and various methods such as substitution plating, electroless plating and pressure reductive plating have been suggested.
The substitution plating, to which the present invention pertains, has been widely utilized as a simple and highly valuable method for plating powders such as metals, metal oxides, metal sulfides, carbonaceous materials, carbides and ceramics with metals such as Au, Pt, Pd, Ag, Cu, Sn, Ni and Co. The generally known substitution methods are roughly classified into a method which comprises adding a solution of a plating metal under stirring to a mixture of a powder to be plated and a reducing metal powder or to these ingredients during their mixing; and a method which comprises adding a powder to be plated and a reducing metal powder together or separately to a solution of a plating metal while stirring the solution. Both of these methods are based on reducing an ion of a plating metal with a powder of a metal which is less noble than the plating metal. It is well known therefore that in the plating of metals having a positive standard electrode potential, such as Au, Pt, Pd, Ag and Cu, both the efficiency of reduction and the plating finish are very good.
On the other hand, in plating metals having a negative standard electrode potential, such as Sn, Ni and Co, especially Ni and/or Co to which the present invention relates, both the efficiency of reduction and the plating finish are very poor. For this reason electroless plating or pressure reductive plating are used almost exclusively in plating metals of a negative standard electrode potential.